1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to an apparatus and method for controlling transmission power, which minimizes power consumption in a terminal.
2. Description of the Related Art
A major wireless communication system, WLAN (Wireless Local Access Network) provides services as available in an existing wired LAN system using a wireless medium as a data transmission medium. The WLAN system can transmit and receive data using minimum circuit connections by utilizing RF (Radio Frequency) technology. Diverse WLAN systems are being developed for ultra high-speed communication services. A major WLAN system is based on the IEEE (Institute of electrical and Electronics Engineers) 802.11a standard. It is to be appreciated that the following description is made in the context of an IEEE 802.11a WLAN communication system.
With reference to FIG. 1, the IEEE 802.11a WLAN communication system will be described.
FIG. 1 is a diagram illustrating the IEEE 802.11a WLAN communication system. Referring to FIG. 1, the IEEE 802.11a WLAN communication system comprises an access point (AP) 110 that services a basic service set (BSS) 100 and a plurality of terminals 120, 130, 140 and 150. The terminals 120, 130, 140 and 150 communicate through the AP 110 in a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) manner. The CSMA/CA is a transmission technique that allows a plurality of terminals to share the same resources, that is, the same carrier, by utilizing a multiple access technique to avoid collision. If the carrier is busy, the terminals 120, 130, 140 and 150 and the AP 110 attempt to transmit data after a random backoff time, thereby preventing collision.
The random backoff time is a length of time to wait until an opportunity to transmit data is obtained. When the carrier is busy, the terminals 120, 130, 140 and 150 and the AP 110 wait for the random backoff time and then attempt to transmit data.
The structure of the IEEE 802.11a WLAN communication system has been described above in connection with FIG. 1. Now, a description will be made of transmit power of the IEEE 802.11a WLAN communication system with reference to FIG. 2.
FIG. 2 is a diagram illustrating signal coverage areas corresponding to the transmission power in the IEEE 802.11a WLAN communication system.
Referring to FIG. 2, the signal coverage area corresponding to the transmission power of the AP 110 is the area of the BSS 100 as marked with a dotted line. The signal coverage area 200 available by the transmission power of the terminal 120 is marked with a solid line. Because a signal from the terminal 120 reaches the other terminals 130, 140 and 150 within the BSS 100, the terminals 130, 140 and 150 normally detect the signal from the terminal 120, thereby avoiding collision. Although the terminals transmission signals with optimum transmit power, thus avoiding collision, an increase in the transmission power increases power consumption in the terminals in the IEEE 802.11a WLAN communication system.
The recent increase of user demand for the IEEE 802.11a WLAN communication system has driven studies on the increase of data rate in the IEEE 802.11a WLAN communication environment. Reduction of power consumption in the terminals through control of transmission power or receiver power is considered as important as increasing the data rate. It is because most terminals such as PDAs (Personal Digital Assistants) other than APs work with batteries and thus power consumption reduction is an important factor that increases the use time of the terminals.
Transmission power control will now be described below.
First, the transmission power itself is reduced, which is described with reference to FIG. 3.
FIG. 3 is a diagram illustrating a power control mechanism based on the transmission power reduction in the IEEE 802.11a WLAN communication system.
Referring to FIG. 3, the transmission power of the AP 110 allows a signal from the AP 110 to cover the BSS 100, as described in connection with FIG. 2. In view of a power control based on transmission power reduction, the signal coverage area of the terminal 120 in correspondence with its transmission power is an area 300 marked with a solid line. The signal coverage area 200 of the terminal 120 illustrated in FIG. 2 is scaled down to the area 300 because the transmission power of the terminal 120 is reduced.
The signal from the terminal 120 only reaches the terminal 140 within the BSS 100. The other terminals 130 and 150 cannot hear the terminal 120, and would thus fail to detect the use of the carrier. As a result, the terminals 130 and 150 may transmit data, causing collision. Terminals that cannot detect signal transmissions from other terminals are called hidden terminals. Since a hidden terminal does not detect the use of the carrier, the terminal attempts a signal transmission while the carrier is busy for another terminal. The resulting collision leads to a signal retransmission. Eventually, power consumption is increased.
Another transmission power control method using RTS/CTS (Ready-To-Send/Clear-To-Send) will be described with reference to FIG. 4.
FIG. 4 is a diagram illustrating an RTS/CTS-based transmission power control mechanism in the IEEE 802.11a WLAN communication system.
The RTS/CTS was proposed to solve the hidden terminal problem encountered with the transmission power reduction-based power control. With the use of the RTS/CTS, although the transmission power of a terminal is reduced, there exist no hidden terminals. Thus, power consumption in the terminal can be reduced. Referring to FIG. 4, the signal coverage area of the AP 110 in correspondence with its transmission power is the area of the BSS 100. Because the transmission power of the terminal 120 is reduced, its signal coverage area 300 is smaller than before the transmission power reduction.
The signal from the terminal 120 only reaches the terminal 140 within the BSS 100. The other terminals 130 and 150 cannot hear the terminal 120, and will fail to detect the use of the carrier. As a result, the terminals 130 and 150 may transmit data, causing collision. That is, the terminals 130 and 150 act as hidden terminals. The use of the RTS/CTS prevents the terminals 130 and 150 from acting as hidden terminals.
While the signal coverage area 300 of the terminal 120 becomes less due to the power reduction, the terminals 130 and 150 are allowed to hear the terminal 120 through the RTS/CTS. When the terminal 120 intends to transmission a signal to the AP 110, it transmits an RTS frame at a maximum transmit power level before transmitting the signal. The RTS frame covers an area 400 in correspondence with the maximum transmission power of the terminal 120.
The AP 110 then transmits a CTS frame as an acknowledgement (ACK) in response for the RTS frame to the terminal 120 at a maximum transmission power level. The CTS frame covers the area 100 in correspondence with the maximum transmission power of the AP 110. Both the RTS and CTS frames contain information related to an action time for the terminal 120 to transmit a frame and time information about the ACK from the AP 110. Therefore, the other terminals 130, 140 and 150 do not transmit signals for the time set in the RTS and CTS frames, thereby avoiding collision. After transmitting the RTS frame and receiving the CTS frame, the terminal 120 transmits the actual signal at a decreased power level.
Despite its ability of preventing the appearance of hidden terminals and avoiding collision albeit allowing power reduction, the RTS/CTS has the distinctive shortcomings of power consumption and time delay due to the transmission and reception of the RTS and CTS frames. Moreover, a carrier is occupied for the transmission and reception of the RTS and CTS frames, thereby decreasing the total throughput of the IEEE 802.11a WLAN communication system.